how to slow down pnuematic piston near end?

[knowlittle]

BACKGROUND:
An air actuator moves an object to target in 0.5 sec. Air pressure 70-80 psi, 25 mm dia piston, travel distance 25 cm. Shock absorber (dash pot) at the end. Object 1 kg.

PROBLEM:
Too much impact at stop even with dash pot. My setup walks around.

QUESTION:
Do they make an intelligent air piston that cuts down air pressure near the end? Or can I fabricate one with electronic control? Difficult to implement?

Thanks.

 

[rb1957]

notknowingmuchmore ...

can you add a relief valve to the actuator to limit the pressure ?

 

[kchan711]

I'm sure someone else on here has a more in-depth background in pneumatics, but I'm sure its possible to lower the pressure based on actuator travel.

For instance, your actuator moves the piston while a position sensor tracks its movement. When the piston travels to a certain point, the sensor output signal trips a relief valve. How fast you drop the pressure determines how fast the actuator slows. You would be able to set the position relief point with a simple adjustment. That's my idea. Best of luck.

Kyle

Kyle Chandler

http://www.chiefengineering.net

 

[handleman]

That's really a bit fast to be moving with pneumatics if you care anything at all about shock. You're moving into servo or even linear motor territory. Then you can control the entire motion profile.

-handleman, CSWP (The new, easy test)

 

[MintJulep]

Is the cylinder double-acting, or spring return?

 

[knowlittle]

Travel time is the most important parameter. I need to move my test object to the detector located at the end within certain time. Initial pressure needs to be where it is now. But once it reaches "the end minus 5 mm", I can cut down air pressure. If this is what you mean, please explain how I can do that with a relief valve. Do I need to install a sensor (a microswitch?) and let the sensor trigger the valve?

 

[knowlittle]

While I was typing, replies were posted. Let me catch up with replies.

REPLY 1: DIFFICULT TO ACHIEVE WITH PNEUMATICS. NEEDS TO BE ELECTRICAL ACTUATOR.

Thanks for the tip. Someone built the setup for me. Is there a drop-in replacement? Can I just remove the pneumatic cylinder with an electrical cylinder? Please advise.

REPLY 2: TRACK POSITION SENSOR OUTPUT

Mine does not have a position sensor. You seem to indicate there are such products available. I will do google search. Thanks.

REPLY 3: CYLINDER DOUBLE ACTING OR SPRING RETURN?

Yes, it is double acting. I don;t quite understand spring return, but if you are asking is there is a spring that pushes the piston back, no, it's not that kind.

Thanks.

 

[MintJulep]

Pneumatics - as you have found - tend to slam things around. With an approximately constant pressure source (as you would have with a sufficiently large regulator)feeding a cylinder you essentially have a constant force input, hence constant acceleration. So the motion-velocity profile is pretty much governed by the equations of linear motion with constant acceleration.

Using a choke, it is possible to get a condition where the cylinder volume changes faster than the supply can fill it. This will result in a reduction in force until the air supply can "catch up". As you might imagine, it's rather difficult to get this set up to work consistently.

Most cylinders have an option for a reed switch or other sensor to detect the piston. Using such a switch, you could cut off your supply a some point, and simply vent the "supply" side of the cylinder and just let the thing coast. You could even achieve braking if you choke the vent. You might be able to make such an arrangement work for you.

 

[DesignerGuy16]

Spring return cylinders will return to a home (extended or retracted) when air is not applied to the inlet port.

They typically only come with 3-6 lb springs. I really wouldn't recommend them as you'll get better life and results from a double actuating cylinder and a 3 position valve.

Cheap and dirty you could get a one way meter-out flow control and throttle it down so it acts like a cushion on extend/retract. So you're extending faster than the exhaust can escape so you compress the air to a certain extent. I'd only try this if your extended position doesn't need to be very accurate.

James Spisich
Design Engineer, CSWP

 

[BrianPetersen]

pneumatics are poor for controlling the motion profile at the best of times, but to help matters, check the following.

Are there any flow controls involved? Normally this will be a little gizmo threaded into the cylinder ports with an adjustment knob on it. It is also possible that it uses in-line flow controls somewhere in the hose between directional valve and cylinder, or (rarely - more common in hydraulics) it may be at the directional valve manifold itself. These restrict the speed of air flow in an adjustable manner, and normally they only restrict in one direction - which should be "out". A "meter-out" arrangement will let the air slowly out of the side of the cylinder being vented and you can adjust it.

If there aren't any flow controls ... you need them. Any pneumatics supplier (Festo, SMC, Numatics, Parker etc) can provide these fittings.

Next question is the type of solenoid valve. There are several types that do different things depending on what you want to have happen when the solenoid(s) is/are turned off. "2 position single solenoid spring return" will have the cylinder return to the home position. "3 position dual solenoid center closed" will stop it dead (although the cylinder will still drift slowly due to unavoidable leakage). "Center exhaust" will basically put the cylinder in "neutral". "Detented" will hold it in the previously-left state (but the bad thing is that if the solenoid power is cut mid-travel, the cylinder keeps going to the end stop). Watch out for possible safety-related implications.

The good thing about "center exhaust" is that it puts the cylinder in a free state when de-energized. Bad thing #1 is that if the motion is vertical, it will now come crashing down by gravity. Bad thing #2 is that even if you have meter-out flow controls, the loss of pressure from both sides will slam the cylinder because there's nothing for the flow control to restrict.

There won't be a perfect solution to this involving pneumatics. Can't be done. It might be possible to be "good enough" or "better than what you've got now", but if really good control of the motion profile is needed, you will have to use either hydraulics or air-over-oil or electric servo control.

 

[zekeman]

You can possibly do it with a bang-bang setup with a double acting cylinder wherein you forward pressurize for 1/2 the stroke and then reverse pressurize for the other half which should get you can open loop to get you to a low velocity near the end then reverse pressurize again, finally lightly impacting a shock absorber.
You will need fast acting valves to evacuate the forward chamber quickly and your forces will be 4x as great since you will accelerate to double the average velocity in 1/2 the time.
It looks pretty manageable to me since you are talking an average velocity of 20"/sec; that means you will achieve 80"/sec with this setup or about 7'/sec. I've seen worse.

 

[SincoTC]

I agree with all the comments regarding the lack of fine control and the "bang bang" nature of most pneumatic devices. Throttling the exhaust side with one way needle valves is better than trying to restrict the inlet as you are controlling the action of the full pressure against the piston rather than trying to prevent this pressure build up, which will happen quicker than you think!

You might care to have a look at SMC's REC Sine Cylinder, it's supposed to give a much kinder "ramped" profile, but it was relatively expensive. We also looked at their Cushion cylinders, but after dabbling with 3-way valves and reed switches, stick-slip effects and gravity was the killer so we opted for a stepper drive.

http://www.smcpneumatics.ie/binary/1/2/25/222/1229/attachment/REC.pdf

Good luck

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2009x32 SP1.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.11.7751

 

[chief]

This type of problem is similar to the old steam reciprocating engines. Where the steam was cut off before end of stroke and allowed to expand and move the piston. A similar arrangement can be applied to a ported shuttle valve to cut off the air before the end of the piston stroke.

Offshore Engineering&Design

 

[griffengm]

Some time back, we used a pneumatic set up on some production lathes to fast forward a tool slide and then decrease to a cutting feed. I'm sorry that I do not remember the device name but it consisted of two cylinders with a sizeable adjustment range for changeover point and final feed rate. Manufacturer's name just came to me - Schrader-Bellows(check spelling).
Hope it helps.
Griffy

 

[MBPServices]

You had quite a lot of replies, that'll get you thinking, no doubt! My opinion is that you'll struggle with some of the suggestions made, as you're only moving 25mm, and moving quite quickly to build in any reduction of force part way through the movement. As someone said, pneumatics do tend to bang things around. With a stroke sensor, you'll only have 0.1-0.2 seconds for something to react, and the pressure to equilibrilise and reduce the force for the last 5mm. You say you need to have 70psi at the start, but don't say why. I would suggest reducing the pressure as much as you can, ie only just enough to move the part, then have a look at your shock absorber. Maybe change it for one with a longer stroke, with higher initial resistance, to give the decelleration you need. This should give you a smoother decelleration than trying to suddenly change the input pressure. Talk to a specialist shock absorber supplier. An incorrectly specified shock absorber is no better not having one at all.
If you do try valves, flow restrictor, etc, keep everything as close to the cylinder as possible, to minimise effected volumes.
If you do use a reed switch or similar to sense the stroke, you will have to move it along the cylinder to allow for response times.
Ring SMC and get their local rep to come and have a look!
Good luck

 

[Pud]

We use sprue-picking robots on moulding machines. These travel 750mm in 0.8s. Towards the end of the stroke, the solenoid is made to cycle on/off very rapidly - I would guess approx 10Hz. (Sounds like pursing your lips and blowing to make a rude noise!) Timers allow the start position of this to be set.
When in "set-up" mode, the solenoid cycles continuously to keep things slow!

But, and here's the thing - the arm is carbon composite lightweight tube with around 200g on the end.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[griffengm]

I returned to your original post and 25 cm travel was very similar to our application of the Schrader-Bellows unit. IIRC, these units came in various configurations to fit a range of applications. They were an integrated actuator and control. We typically operated them through a solenoid.
Griffy

 

[MechEng2005]

Don't know if it's possible for your application, but why not add a spring at the "target" area to slow it down a bit as it gets close? If it works, this may be easier than a more advanced (i.e. position sensing) cylinder or internal customizations to the cylinder.

Also, the spring would be external and easy check for damage or replace...

-- MechEng2005

 

[rb1957]

as i said at 1st, notknowingmuchmore ...

as i understand it you've got a 70psi airsupply driving an actuator that's wacking into a 1kg target trying to move it 25cm and your set-up is "walking all over the place".

is this like a brillards/snooker robot ? so you can't "nail" your set-up to the ground ??

how much work are you trying to do ? what sort of friction is under the target ??

how precise is the target's final position ?

could you use an upstream reserviour, to lower and control the airsupply pressure ?

can the actuator extend 25cm ? so you can positively place the target where you want it ??

 

[alansimpson]

Most pneumatic actuator manufacturers produce cylinders with adjustable cushioning on ends. Try Festo or SMC. SMC has a tapered arrangement which throttles the air at end of stroke. For such high speeds a high flow fast reacting valve could be used. I have used direct acting poppet valve manufactured by MAC. Using separate valves for advance and return and getting the timing right the return valve could cushion end of stroke. Normal pneumatic valves use pilot valves to actuate main valve and I have found this to be too slow for fast response.
For real speed control linear motor is the way to go. SMAC,

http://www.trilogysystems.com/,http://www.linmot.com/lm_typo/index.php